Question

Can you help me to find a simple tutorial of how sign a string using ECDSA algorithm in java. But without using any third-party libraries like bouncycastle. Just JDK 7. I found it difficult to search a simple example, I’m new to cryptography.

---

import java.io.*;
import java.security.*;

public class GenSig {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        /*
         * Generate a DSA signature
         */

        try {

            /*
             * Generate a key pair
             */

            KeyPairGenerator keyGen = KeyPairGenerator.getInstance("DSA", "SUN");
            SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");

            keyGen.initialize(1024, random);

            KeyPair pair = keyGen.generateKeyPair();
            PrivateKey priv = pair.getPrivate();
            PublicKey pub = pair.getPublic();

            /*
             * Create a Signature object and initialize it with the private key
             */

            Signature dsa = Signature.getInstance("SHA1withDSA", "SUN");

            dsa.initSign(priv);

            String str = "This is string to sign";
            byte[] strByte = str.getBytes();
            dsa.update(strByte);

            /*
             * Now that all the data to be signed has been read in, generate a
             * signature for it
             */

            byte[] realSig = dsa.sign();
            System.out.println("Signature: " + new String(realSig));


        } catch (Exception e) {
            System.err.println("Caught exception " + e.toString());
        }
    }
}

How to modify it for ECDSA?

Answer 1

Here is small example based on your example.

NOTE: this is the original code for this answer, please see the next code snippet for an updated version.

import java.math.BigInteger;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.security.Signature;

public class ECDSAExample {

    public static void main(String[] args) throws Exception {
        /*
         * Generate an ECDSA signature
         */

        /*
         * Generate a key pair
         */

        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("EC");
        SecureRandom random = SecureRandom.getInstance("SHA1PRNG");

        keyGen.initialize(256, random);

        KeyPair pair = keyGen.generateKeyPair();
        PrivateKey priv = pair.getPrivate();
        PublicKey pub = pair.getPublic();

        /*
         * Create a Signature object and initialize it with the private key
         */

        Signature dsa = Signature.getInstance("SHA1withECDSA");

        dsa.initSign(priv);

        String str = "This is string to sign";
        byte[] strByte = str.getBytes("UTF-8");
        dsa.update(strByte);

        /*
         * Now that all the data to be signed has been read in, generate a
         * signature for it
         */

        byte[] realSig = dsa.sign();
        System.out.println("Signature: " + new BigInteger(1, realSig).toString(16));

    }
}

UPDATE: Here is slightly improved example removing deprecated algorithms. It also explicitly requests the NIST P-256 curve using the SECG notation "secp256r1" as specified in RFC 8422.

import java.math.BigInteger;
import java.security.*;
import java.security.spec.ECGenParameterSpec;

public class ECDSAExample {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws Exception {
        /*
         * Generate an ECDSA signature
         */

        /*
         * Generate a key pair
         */

        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("EC");

        keyGen.initialize(new ECGenParameterSpec("secp256r1"), new SecureRandom());

        KeyPair pair = keyGen.generateKeyPair();
        PrivateKey priv = pair.getPrivate();
        PublicKey pub = pair.getPublic();

        /*
         * Create a Signature object and initialize it with the private key
         */

        Signature ecdsa = Signature.getInstance("SHA256withECDSA");

        ecdsa.initSign(priv);

        String str = "This is string to sign";
        byte[] strByte = str.getBytes("UTF-8");
        ecdsa.update(strByte);

        /*
         * Now that all the data to be signed has been read in, generate a
         * signature for it
         */

        byte[] realSig = ecdsa.sign();
        System.out.println("Signature: " + new BigInteger(1, realSig).toString(16));

    }
}